Noise reduction in an air moving apparatus

ABSTRACT

An air moving apparatus for generating cooling airflow is provided that includes a noise reduction system for reducing noise generated by a fan. The air moving apparatus includes a fan having a rotatable hub and a plurality of blades mounted to the hub for rotating about an axis of rotation to provide pressurized airflow. A sensor is situated on a surface of at least one fan blade for sensing airflow characteristics of the air flowing over the fan blade. An actuator, also situated on the surface of the fan blade, changes the characteristic of the airflow over the fan blade in response to the sensed airflow characteristic.

FIELD OF THE INVENTION

The present invention relates to an air moving apparatus and, moreparticularly to fans having low-noise characteristics and a method foractively optimizing such fan characteristics.

BACKGROUND OF THE INVENTION

A wide variety of equipment and systems, such as portable and desktopcomputers, mainframe computers, communication infrastructure frames,automotive equipment, etc., include heat-generating components in theircasings. As increasingly dense and higher performance electronics arepackaged into smaller housings, the need for effective cooling systemsis paramount to prevent failure of such sensitive electronics devices.One method used to remove heat from such equipment is to have an axialfan draw air from the exterior of the casing to blow cooling air overthe heat-generating components. However, as the number of electronicsdevices in offices and households increase, so too does the number ofcooling fans. As such, fan noise becomes significantly loud andundesirable.

Noise reduction in fans generally is accomplished through either activeand/or passive noise reduction techniques. In a passive noise reductionsystem, a fan may include a plurality of projections having a number ofpredetermined masses that are arranged at positions around the peripheryof the blade. This results in creating an unstable mode for the fan. Theunstable mode results in disruption of airflow over the blade, therebyresulting in less noise at the trailing edge. However, such a systemrequires the fan to rotate at a preset rotational speed for maximumeffectiveness. Rotation of the fan at other than the preset speedresults in decreased effectiveness of the noise reduction methods.

An active noise reduction method includes a fan having a micro electromechanical system that includes a thin silicon film forming anintegrated circuit and an actuator connected to the circuit forgenerating vibrations. The fan reduces noise by causing the actuator togenerate vibration that offsets or reduces unstable airflow along theblade body. However, the operation of the noise reduction system is lessthan optimal because the actuator and the sensing portion are configuredas a closely spaced, or even single, device that is placed at oneparticular portion of the fan blade. Thus, the actuator and the sensingportion are separated by a negligible distance. As such, the system isunable to simultaneously sense the wake at the trailing edge of theblade and create turbulent flow at a predetermined point along the fanblade.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an airfoil illustrating the principles ofvortex shedding;

FIG. 2 is a perspective view of a fan having noise reductioncapabilities in accordance with the invention;

FIG. 3 is a side view of a fan blade of the fan of FIG. 2 having asensor and actuator mounted thereon in accordance with the invention;

FIG. 4 is a perspective view of the back side of the fan of FIG. 2having a controller mounted thereon in accordance with the invention;and

FIG. 5 is a flow diagram of the controller in operation in accordancewith the invention.

DETAILED DESCRIPTION

A known problem with axial fans relates to vortex shedding, which is theprinciple contributor of aero-acoustic noise in fan operation. Referringto FIG. 1, the mechanism of vortex shedding is shown. In a fan thedirection of airflow 13 is partly over the surface of an axial fan blade11 from the leading edge 16 to the trailing edge 19 of the airfoil of apressure gradient. At the leading edge 16 of the airfoil and up to acertain distance along the blade 11, the flow of air is laminar 18. Thatis, there is smooth, uninterrupted flow of air over the surface contour12 of the fan blade 11. This air flow forms a boundary layer since theair flow has zero velocity right at the surface, and some distance outfrom the surface it flows at the same velocity as the local outsideflow. If the boundary layer flows in parallel layers, with no energytransfer between layers, it is laminar. If there is energy transfer,airflow is no longer laminar, but turbulent 17. All boundary layersstart off as laminar. However, due to adverse pressure gradient surfaceroughness and other destabilizing influences, the airflow 13 begins toseparate from the surface 12 of the airfoil blade 11 after a certaindistance along the length of the airfoil blade 11. As a result, thepressure and flow becomes more mixed and turbulent, with an increase inthe radial or drag direction. The point at which the airflow becomesturbulent is known as a transition regime 15.

As air flows past the trailing edge 19 of the blade 11, it generates awake 23 behind the blade 11. This is caused by the pressure gradientbeing in the opposite direction to the airflow. Therefore an eddy or airvortex 21 is created behind the trailing edge of the fan. A similareffect takes place with the airflow around the bottom side 14 of the fanblade 11. These air vortices drop off the back of the fan blade creatingthe wake 23 behind the blade. This effect is known as vortex shedding.Vortex shedding 21 in this wake region 23 causes pressure fluctuationresulting in generation of acoustic waves and other unwanted vibration.These acoustic waves create noise when the fan is operating.

Referring to FIG. 2, there is illustrated an air moving apparatus in theform of a tube-axial fan 37 in accordance with the present inventionhaving increased noise reduction capabilities via the provided sensors27 operating in concert with actuators 31 on the fan blade 25 of the fan37. The frequency of the oscillation of the actuator 31 for decreasingfan noise is dynamically determined from acoustic input received by thesensor 27 and actively adjusted by a controller 41 (FIG. 4) as desiredfor quiet operation. In this manner, the present fan 37 is particularlyeffective in those applications where the fan noise may be excessive,i.e. small casings enclosing high-density consumer electronics therein.

The fan 37 includes a plurality of fan blades 25 extending generallyradially outward from a hub 38. Each fan blade 25 terminates at a tipend portion 28 thereof radially spaced from the hub 38 and has a leadingedge 16 and a trailing edge 19 extending between the hub 38 and the tipend portion 28. The fan is rotatively driven by an output shaft of amotor (not shown) that engages the center 39 of the hub 38. The motorrotates the fan 37 about a central longitudinal axis that is defined bythe receiving portion 39 of the fan 37. This causes the fan blades 25 todraw air from an inlet side 26 of the fan 37 and to impart velocity todischarge the air from an outlet side 29 in the direction generallyindicated by arrow 34.

Turning to FIG. 3, the fan blade 25 of the fan 37 in accordance with thepresent invention is shown in greater detail. The fan blade 25 has abottom side 35 and a top side 33. The top side 33 has mounted thereon apiezoelectric sensor element 27 made of thin organic polymer such aspolyvinylidene fluoride (PVDF) or lead zirconate titanate (PZT). Using,for example, the PVDF piezoelectric sensor element 27 on the trailingedge 19 of the fan blade provides several significant advantages oversensors made of thin film silicon or the like. For example, the PVDFsensor material is an inexpensive thin plastic polymer sheet or filmthat has a thin electrically conductive nickel copper alloy deposited oneach side. Electrical connections are made to the film using wires thatmay be attached to the conductive coating of the film using copper tapeor conductive epoxy. The film itself may be cut to shape as needed andglued onto the appropriate location on the fan blade 25. Thus, theadvantages of using the PVDF sensor include its low cost and the ease inwhich the sensor may be configured for use in a variety of fan bladesizes.

The sensor element 27 is attached on the trailing edge of the blade andsenses pressure fluctuation and acoustic energy at the trailing edge ofthe blade 25. Fluctuations in air pressure are detected by the sensor 27when air pressure or sound waves, such as acoustical waves, cause thefilm to stretch and conduct electricity, thereby creating a closedcircuit between the wires. The system of the present invention detectsthe closing and opening of the circuit to determine characteristics ofthe waves at the trailing edge of the blade 25. Thus, the sensor is ableto determine the presence of noise causing air waves.

The top side of the fan blade 25 also has mounted thereon an actuator 31made of piezoelectric element and a thin layer or fin 30 attached on thetop surface. The actuator 31, being also made of piezoelectric film, ismade to vibrate, which in turn causes the fin 30 to vibrate as well.Applying and removing voltage to the fin 30 causes the material to bendand then return to its original shape, thereby creating a vibrationmotion. Alternatively, two sheets of film maybe joined together to forma bimorph. The sheets are arranged such that when voltage is applied tothe bimorph, one film laminate lengthens while the other contracts.Voltage of the reverse polarity causes the bimorph to bend in the otherdirection. Thus, the vibration rate of the actuator is controlled in thefirst case by pulsing power to the film or in the second case byreversing the polarity of the voltage being supplied to the bimorph.

As shown, the sensor element 27 and actuator 29 are purposefully spacedapart. An advantage of such a configuration is the ability to detectnoise in the area of the fan where most noise originates, i.e. thetrailing edge, and to correct or eliminate the conditions that lead tothe noise by creating turbulence in the laminar flow region. As such,fan noise caused by vortex shedding is reduced through the eliminationof the shedding of vortices by deliberately converting laminar flow toturbulent flow.

Referring to FIGS. 4 and 5, a controller 41 comprising a feedbackcontrol loop is shown mounted on the hub 43 on the reverse side of thefan 25. The controller hardware may comprise a 16 bitanalog-to-digital/digital-to-analog converter (ADC/DAC), such as theTMC320C62 digital signal processor (DSP), available from TexasInstruments Corporation.

The controller 41 includes an adaptive controller 45 and an actuatorcontroller 47 that is used for exciting the actuator by pulsing thevoltage or controlling the voltage polarity. The feedback control loopof the controller 41 is mounted on the hub 43 of the fan 25 and receivespower and signal from the rotating shaft of the fan. During operation ofthe fan 37, the airflow over the fan blade 25 is laminar near theleading edge 16, and changes to transition regime downstream. Thetransition of boundary lair from laminar regime occurs generally on thesuction side (upper side) 33 of the airfoil blade 25. Based on theacoustic feedback from the sensor 27 at the trailing edge, the actuatorcontroller 47 causes excitation of the boundary layer at a particularpredetermined frequency using the piezoelectric actuator 31 to vibratethe fin 30 at the appropriate frequency as determined by the adaptivecontroller 45. Thus, the laminar airflow is converted to turbulent flowdeliberately. Accordingly, the problems of noise associated with thetransition to turbulent flow and subsequent vortex generation isreduced.

Continuing to refer to FIG. 5, the control loop is shown in operation.As discussed above, the acoustic wave emitted from the blade 25 has aparticular frequency spectrum. The sound pressure level at the trailingedge 27 is a function of the aerodynamic loading, speed, and the inletturbulence level. The frequency spectrum also changes in a similarmanner. Based on the acoustic input at the sensor 27, the controlcircuit 41 (FIG. 4) determines the required frequency of thepiezoelectric actuator 31. In particular, the control loop determinesthe sound pressure level versus the frequency data from the sensor 27input in narrow band over a period of time. The control loop then scalesthe data using a preset scale, such as A scale, of acoustic averaging.From the scaled sound pressure data, the control loop determines theobjectional frequency peaks, such as 1000 Hz, or any other objectionablefrequency in the audible range of human bearing.

The piezoelectric actuator 31 causes vibration on one end of the fin 30.The fin 30 vibrates, generating pressure fluctuation on the surface ofthe airfoil blade 25. The pressure fluctuation results in breakup of theattached laminar flow. This causes the laminar flow to transition toturbulent flow early and before reaching the trailing edge 19, resultingin reduced or eliminated vortex shedding and correspondingly lowerednoise levels. The amount of vibration required of the fin is adaptivelydetermined by the controller 41. In particular, the feedback controlloop of the controller 41 determines frequency windows for generatingcorrection signals. Depending on the level of turbulence generated theacoustic wave radiation at the trailing edge 19 changes.

Based on the change of the acoustic wave radiation sensed by thepiezoelectric sensor 27, a control signal modifier or error signal 46 isgenerated. The generated error signal 46 is combined with the predefinedactuator signal 49 to send a corrected signal 50 to the actuator 31. Theactuator control in the feedback loop creates the voltage signal to theactuator 31. The resultant acoustic signal from this correction is againreceived from the sensor 31 and the above process is repeated untilcancellations of the objectionable sound pressure peaks are eliminated.Thus, an active control loop is established. Accordingly, the controlcircuit automatically and dynamically establishes the appropriate signalfor the actuator depending in the change in loading or any otherparameter changes.

While there have been illustrated and described particular embodimentsof the present invention, it will be appreciated that numerous changesand modifications will occur to those skilled in the art, and itintended in the impendent claims to cover all those changes andmodifications that fall within the true spirit and scope of the presentinvention.

1. In an air moving apparatus for generating cooling air flow, a noisereduction system comprising: a fan having a rotatable hub and pluralityof blades mounted to the hub for rotating about an axis of rotation toprovide pressurized airflow each of the blades having a trailing edgeand a leading edge; a sensor of at least one fan blade positioned forsensing a wake at the trailing edge of the one fan blade; and anactuator between the leading and trailing edge of the one fan blade andspaced from the sensor to generate turbulent airflow over the fan bladeat a position where airflow is otherwise laminar in response to thesensed wake at the trailing edge.
 2. The noise reduction system of claim1, further comprising a controller for receiving data from the sensorand enabling the actuator to vibrate at a frequency based on thereceived sensor data.
 3. The noise reduction system of claim 1, whereinthe sensor is a piezoelectric sensor.
 4. The noise reduction system ofclaim 3, wherein the piezoelectric sensor is located on the trailingedge of the fan blade for sensing pressure fluctuation at the trailingedge.
 5. The noise reduction system of claim 3, wherein thepiezoelectric sensor is located on the trailing edge of the fan bladeand senses acoustic energy at the trailing edge.
 6. The noise reductionsystem of claim 1, wherein the actuator is a piezoelectric element withan attached thin layer fin.
 7. In an air moving apparatus for generatingcooling air flow, a noise reduction system comprising: a fan having arotatable hub and plurality of blades mounted to the hub for rotatingabout an axis of rotation to provide pressurized airflow; a sensorsituated on a surface of at least one fan blade for sensing at least onecharacteristic of airflow over the fan blade; and an actuator situatedon the surface of the fan blade for changing the characteristic ofairflow over the fan blade in response to the sensed airflowcharacteristic, wherein the vibration frequency of the actuator isdetermined using data provided by the sensor to convert laminar airflowto turbulent airflow for reducing vortex shedding, the actuator enablingthe fin to vibrate at a predetermined frequency to cause laminar airflow to become turbulent air flow before reaching a trailing edge of thefan blade.
 8. A method for reducing noise in an air moving apparatus,comprising the steps of: generating pressurized airflow using one ormore fan blades; sensing at least one characteristic of the airflow at atrailing edge of one of the fan blades; and changing the characteristicof the airflow upstream of the trailing edge in response to the sensedairflow characteristic at the one fan blade trailing edge.
 9. The methodof claim 8 wherein the sensing step further comprises the step ofsensing with a piezoelectric sensor the acoustic energy at the trailingedge of the fan blade.
 10. The method of claim 8 wherein the sensingstep further comprises the step of sensing with a piezoelectric sensorthe pressure fluctuation at the trailing edge of the fan blade.
 11. Amethod for reducing noise in an air moving apparatus, comprising thesteps of: generating pressurized airflow using one or more fan blades;sensing at least one characteristic of the airflow as it travels overthe fan blade at a sensing location; and changing the characteristic ofthe airflow in response to the sensed airflow characteristic, whereinthe characteristic changing step further comprises the step ofconverting laminar air flow to turbulent air flow to prevent vortexshedding on the trailing edge of the fan blade.
 12. The method of claim11 wherein the converting step further comprises the step of vibrating apiezoelectric actuator at a predetermined frequency at a predeterminedlocation on the fan blade.
 13. The method of claim 12 wherein theconverting step further comprises generating a control signal specifyingthe rate of vibration of the piezoelectric actuator.
 14. The method ofclaim 13 further comprising the step of creating a feedback control loopto dynamically control the frequency rate of the piezoelectric actuator.15. The method of claim 14 wherein the feedback control loop creatingstep further comprises the steps of generating a control signal modifierbased on the sensed characteristic and combining the control signalmodifier with the control signal to dynamically create a new controlsignal specifying the rate of vibration of the piezoelectric actuator.16. The method of claim 12 further comprising the step of calculatingthe rate of the frequency based on the sensed airflow characteristic.17. A noise reduction system for an air moving apparatus comprising:means for generating pressurized airflow including a plurality of bladeseach having a trailing edge and a leading edge; means for sensing atleast one characteristic of the airflow over at least one of the blades,the sensing means located on the generating means; and means forchanging a second characteristic of the airflow over the one fan bladein response to the sensed airflow characteristic, the changing meanslocated on the generating means spaced from the sensing means, whereinthe changing means changes the second characteristic of airflow to theone characteristic of airflow to reduce noise created by operation ofthe generating means.
 18. The noise reduction system of claim 17 furthercomprising means for calculating operating parameters and controllingoperation of the dynamically varying means based on the calculatedoperating parameters.
 19. A noise reduction system for an air movingapparatus comprising; means for generating pressurized airflow; meansfor sensing at least one characteristic of the airflow over thegenerating means, the sensing means located on the generating means; andmeans for changing the characteristic of the airflow over the fan bladein response to the sensed airflow characteristic, the changing meanslocated on the generating means, the sensing means located at asignificant distance from the change means, wherein the characteristicchanging means comprises means for converting laminar airflow over thepressurized airflow generating means into turbulent airflow.
 20. Thenoise reduction system of claim 19 further comprising means fordynamically varying the operation of the converting means in response tothe sensed airflow characteristics.